FAT Boot Sector


This paper describes the FAT boot sector. I will try to unify the boot sectors of the FAT32, FAT16, and FAT12 file systems.


Boot sector is always the very first sector in the partition. Validity check is performed by comparing the 16 bit word at offset 1FE to AA55. For FAT systems, this sector always contains the Bios Parameter Block (BPB) at offset 0B. The structure of the boot sector is below.
Offset in Boot Sector Length in Bytes Mnemonic
03 8 OEM_Identifier
0B 2 BytesPerSector
0D 1 SectorsPerCluster
0E 2 ReservedSectors
10 1 NumberOfFATs
11 2 RootEntries
13 2 NumberOfSectors
15 1 MediaDescriptor
16 2 SectorsPerFAT
18 2 SectorsPerHead
1A 2 HeadsPerCylinder
1C 4 HiddenSectors
20 4 BigNumberOfSectors
24 4 BigSectorsPerFAT
28 2 ExtFlags
2A 2 FSVersion
2C 4 RootDirectoryStart
30 2 FSInfoSector
32 2 BackupBootSector
34 12d Reserved

OEM_Identifier is the eight-byte ASCII string that identifies the system that formatted the disk. All eight characters are meaningful. Spaces or zeroes are appended if the name is less than eight characters long. As any other OEM name, this string is nice to display near to the other disk information, but it is absolutely useless for any other purpose.

BytesPerSector is how many bytes long the physical sector is. All disks I have ever seen contained 512 in this field. The gossips are, DOS supports disks with different sector sizes. Other gossips are, early versions of DOS silently set this field to 512 and ignored whatever the original value was. Anyway, as of today, you will not loose much if you compare this value to 512 and refuse to work with the disks that have a different sector size.

SectorsPerCluster is how many sectors are in one logical cluster. What is cluster was described earlier. This value should not be zero.

ReservedSectors are reserved, starting from the LBA sector (0) relative to current partition. LBA (ReservedSectors) is the beginning of the first FAT. For FAT12 and FAT16 this value is usually 1. For FAT32 it is 20h. At least one sector must always be reserved.

NumberOfFATs is the number of File Allocation Tables. This value is usually two. FATs are consecutive on the disk: the second copy of FAT goes right after the first copy. At least one copy of FAT should be present.

RootEntries contains the number of the entries in the root directory if root directory is fixed. It is zero if the root directory is not fixed. FAT32 disks should contain zero in this field, indicating that the root directory can be arbitrarily long. Otherwise, this field usually contains 512. Each directory entry takes up 32 bytes. To avoid wasting space, RootEntries*32 should be divisible by BytesPerSector.

NumberOfSectors is the total number of sectors on the disk. If the number of sectors is greater than 65535, then this field is set to zero and the dword at BigNumberOfSectors contains the actual number of sectors. By NumberOfSectors I will refer to that of NumberOfSectors and BigNumberOfSectors, which is used. Note that this field should contain the same or lesser value as the corresponding field in the partition table. If the values are not equal, the lesser of them should be used. NumberOfSectors should be large enough to contain at least the reserved sectors, all FAT copies, and the root directory, if any. Disk layout is described below.

MediaDescriptor describes the device:
Value DOS version Meaning
FF 1.1 5 1/4 floppy, 320KB
FE 1.0 5 1/4 floppy, 160KB
FD 2.0 5 1/4 floppy, 360KB
FC 2.0 5 1/4 floppy, 180KB
F9 3.0 5 1/4 floppy, 1.2MB
F9 3.2 3 1/2 floppy, 720KB
F8 2.0 Any Hard Drive
F0 3.3 3 1/2 floppy, 1.44MB

As of today, no other values are defined.

SectorsPerFAT contains the number of sectors in one FAT. This field is zero for FAT32 drives, and BigSectorsPerFAT contains the actual value. Note that the Microsoft FAT32 boot loader will not work with the FAT32 drives if SectorsPerFAT is not zero, and FAT12 and FAT16 loaders will not work with the drives if SectorsPerFAT is zero. As with NumberOfSectors, by SectorsPerFAT I will mean the appropriate value. It goes without saying, FAT should be long enough to contain the information about all clusters on the disk.

SectorsPerHead is the number of sectors grouped under one head. HeadsPerCylinder is also what you think it is. If this partition is a CHS partition, these values must be the same as those returned by BIOS. If they are not the same, the disk was misconfigured and the partition should not be used. Note that the Microsoft boot loader alteres the BIOS Diskette Parameters table by setting the SectorsPerTrack field of this structure to SectorsPerHead read from the boot disk. The values in these fields do not matter for LBA partitions.

HiddenSectors is the number of sectors between the beginning of this partition and the partition table. This field should be the same as "number of sectors preceding the partition" in the partition table. Note that it is not necessarily the physical LBA address of the first sector because there exist secondary partitions. If HiddenSectors is not the same as in the partition table, boot sector was corrupted and the partition should not be used. Also note that the high word contains garbage for old versions of DOS.

ExtFlags, and all fields described below, are defined only for FAT32 disks. They are defined differently for FAT12 and FAT16 (that issue is discussed below). If the left-most bit of ExtFlags value is set then only the active copy of FAT is changed. If the bit is cleared then FATs will be kept in synchronization. Disk analyzing programs should set this bit only if some copies of the FAT contain defective sectors. Low four bits define which copy should be active. As you see, only the first sixteen copies of the FAT may be selected active, so the disk is usable if and only if among the first sixteen copies at least one is usable. For this flag, FAT numbers start from zero. Sanity check insures that the active FAT is less than NumberOfFATs.

FSVersion is the version of the file system. The high byte is the major version, the low byte is the minor version. Both are set to zero on my Windows 95 OSR2. I do not think that this value should be checked. However, the Microsoft boot loader does check it in certain cases, and it complains if it is not zero.

RootDirectoryStart contains the number of the first cluster for the root directory. Yes, finally the root directory became stored like any other directory, in the cluster chain. This also implies that it may grow as needed. The value in this field should be at least two. It is two on my system.

FSInfoSector is the sector number for the file system information sector. This sector is new to FAT32. Its structure is below:
Offset in Sector Size Meaning
00 4 Signature, should be 41615252h (?)
1E4 4 Signature, should be 61417272h
1E8 4 Number of free clusters on the drive, or -1 if unknown
1EC 4 Number of the most recently allocated cluster
1F0 12d Reserved
1FE 2 Signature AA55

All the other bytes are set to zero. As you see, these values are introduced to improve performance. Make sure that FSInfoSector is at least one and it lies within the reserved disk area. Also make sure that this value is not the same as BackupBootSector. If it does not satisfy these conditions, do not use this sector, but the file system should still be usable. Do not use the information in this sector if its signature is incorrect. Note that only the second signature is documented by Microsoft. Normally, the number of free clusters is checked only by special disk analysis programs. FSInfoSector is usually one.

BackupBootSector is the sector number for the backup copy of the boot sector. This copy can be used if the main copy was corrupted. It is also nice to compare the two copies on startup. If they do not match, a warning should be issued. They may not be in tact because of corruption or a boot virus. If this field contains zero or the number greater than or equal to ReservedSectors or the same value as FSInfoSector, the backup sector should not be used.


Note that some of the values in the boot sector have a different meaning in FAT12/16 systems as compared to the FAT32 system. These fields form the Extended BIOS Parameter Block, or EBPB. EBPB is the same for FAT12/16 and FAT32, but it starts from the different offsets in the boot sector. For the sake of completeness I will describe it here:
Offset for FAT12/16 Offset for FAT32 Length in Bytes Meaning
24 40 1 BIOS drive number
25 41 1 Reserved
26 42 1 Extended Boot Record signature = 29h
27 43 4 Serial Number
2B 47 11 Volume label
36 52 8 System Identifier (FAT12, FAT16, or FAT32)

In my humble opinion, you do not need these fields even for FAT12 or FAT16. However, there is one compatibility issue. If you have detected that FAT12 or FAT16 is used, and the extended boot record signature is not 29h, you should ignore all the values in the BPB starting from the offset 1E. This is not a typo: HiddenSectors is really split by half, and if the extended boot record signature is incorrect then only the lower word of HiddenSectors is valid.

Sanity check might also insure that System Identifier is correct for the detected file system. It should be "FAT12", "FAT16", or "FAT32" according to the file system. Strings are padded with spaces to fit in eight bytes.

Finally, the presense of the EBPB in FAT32 is not documented by Microsoft.

Now it is high time to explain how clusters are mapped to sectors. First, consider the layout of the FAT disk:
LBA Location Length in Sectors Description
0 ReservedSectors Boot Sector(s), File System Info Sector
ReservedSectors NumberOfFATs*SectorsPerFAT File Allocation Tables
RootStart * (RootEntries*32)/BytesPerSector Root Directory, if any
ClustersStart NumberOfClusters*SectorsPerCluster Data Clusters
* Note: RootStart does not make sense for FAT32 partitions. However, the formulae below are still valid for FAT32.

Some of the values in the table are in the BPB. Let us calculate the rest of them:

ClustersStart=RootStart+(RootEntries*32) div BytesPerSector
* Note: if (RootEntries*32) mod BytesPerSector then ClustersStart=ClustersStart+1
NumberOfClusters=2+(NumberOfSectors-ClustersStart) div SectorsPerCluster

To convert cluster address to LBA address use the formula:


Now we are ready to detect which file system is used.

I fairly warn you: I have not yet found two sources that agree on these values. So, be careful if the number of clusters is close to the border value. One might have noticed that the maximum NumberOfClusters for FAT32 looks odd. Since only 28 of 32 bits are currently used, the FAT32 partition can have no more than 268,435,456 clusters. Looking at NumberOfClusters is the only recommended by Microsoft way of detecting the FAT entry size.

Since the type of the file system depends on the cluster size, which can be set arbitrarily by the formatting program, let me introduce the Microsoft recommendations. Note that these are only guidelines. The partition size is not used directly to determine the FAT type. It is first converted to NumberOfClusters as described above.
Partition Size Type of FAT
10MB or less FAT12
10MB through 512MB (?) FAT16
512MB (?) through 2TB FAT32
The following table contains the ranges for the partition sizes. They do overlap.
Min Partition Size Max Partition Size Type of FAT
1.5KB 510.75MB FAT12
2.0435MB 8190.75MB FAT16
~32MB 32TB FAT32

Author:  Alex Verstak  3/10/1998